Halogenated hydrocarbons, in particular chlorinated hydrocarbons, by virtue of their solvating power, their non-inflammability and their relatively low boiling point, are used extensively for degreasing metals.
Among the chlorinated hydrocarbons, methylene chloride proves has proven to be particularly stable to oxidation, to hydrolysis and to pyrolysis. Its low boiling point also permits it to be used at low temperatures.
All these properties consequently make it a solvent of choice for degreasing, cleaning and/or drying metal components.
However, when methylene chloride is used in degreasing, cleaning and/or drying some metal components, it can react in particular with aromatic compounds introduced during the machining of the said metal components, e.g., toluene.
This reaction occurs particularly when the said metal components are those made of aluminum or its alloys and those made of magnesium or its alloys.
Without being committed to any one explanation, this reaction is reflected by a more or less significant decomposition of the methylene chloride with the formation, in particular, of acidic compounds such as hydrochloric acid or phosgene.
This decomposition results in a significant decrease in the solvent property of methylene chloride and a significant modification of the pH.
The acid compounds released can additionally corrode the metal components and the plant.
This decomposition is still more marked when methylene chloride is used for hot degreasing or drying operations.
This decomposition of methylene chloride consequently constitutes a serious disadvantage as regards its use as a degreasing, cleaning and/or drying solvent for metal components composed of so-called light metals such as aluminum or magnesium (or their alloys).
Thus, in order to overcome these disadvantages, a great number of compounds have been recommended as inhibitors of the decomposition, or stabilizers, of methylene chloride.
Mention may be made, among the most used compounds, of saturated and unsaturated alcohols, ketones, epoxides, amines, ethers and their mixtures.
Thus, U.S. Pat. No. 3,923,912 recommends the use of methyl ethyl ketone.
U.S. Pat. No. 3,670,036 describes a methylene chloride composition stabilized by:
1 to 10% of a nitroalkane having from 1 to 6 carbon atoms, PA1 0.1 to 5% of an alkylene oxide having from 2 to 4 carbon atoms, and PA1 0 to 10% of a dialkoxyalkane having up to approximately 20 carbon atoms.
Although the use of nitroalkanes introduces a certain preventive effect against the decomposition of methylene chloride in contact with metals, this prevention is very unsatisfactory when it is desired to protect not only the liquid phase but also the vapor phase.
In addition, many compounds mentioned above, used alone or in combination, do not make it possible to pass the very strict long-term stabilization test known as the BAM (Bundesanstadt fur Materialprufung) test.